The present invention relates in general to substrate materials for microfabrication, and in particular to the manufacture of an amorphous silica substrate or portion from the oxidation of a monocrystalline silicon substrate or portion.
Commercially-available fused silica substrates are being considered for use in the manufacture of fluidic devices with integrated optics. These microfluidic devices require that one or more fluidic channels be formed in the substrate. Micron-sized channels (also termed microchannels) can be formed using conventional integrated circuit (IC) processing techniques such photolithographic masking and plasma etching (e.g. reactive ion etching). However, the presence of inclusions (e.g. voids, defects and microcrystallites) in commercially-available fused silica substrates results in nonuniform etching and the production of etch pits which degrade the quality of the etched channels. Furthermore, crystalline silica substrates (i.e. crystalline quartz) are birefringent, rendering these substrates unsuitable as alternatives for the formation of microfluidic devices wherein optical activation or detection is to be used. What is needed are amorphous silica substrates that are homogeneous and that are substantially free of inclusions. Such amorphous silica substrates can be formed according to the present invention by starting with a monocrystalline silicon substrate and oxidizing the silicon substrate material to convert it to silica (i.e. silicon dioxide).
Additionally, for certain optical applications windows are needed through a silicon substrate which is not transparent to visible light. The present invention provides a solution to this problem by allowing the formation of a transparent silica portion from a monocrystalline silicon portion of a substrate.
The present invention relates to a method for forming an inclusion-free silica substrate, comprising steps for providing a monocrystalline silicon substrate, and forming the silica substrate from the silicon substrate by heating the silicon substrate to an elevated temperature and contacting both major surfaces of the silicon substrate with an elevated pressure of an oxidizing ambient, thereby thermally oxidizing the entire silicon substrate. The monocrystalline silicon substrate can be, for example, 10-50 microns (xcexcm) thick. The elevated temperature can be in the range of 700-1200xc2x0 C., with the elevated pressure generally being in the range of 5-50 atmospheres. When using a commercial high-pressure oxidation system designed for forming gate and field oxides, the elevated temperature can be, for example, 900-1000xc2x0 C., with the elevated pressure being generally in the range of 10-25 atmospheres. The oxidizing ambient can comprise oxygen, steam (formed from water or from the reaction of hydrogen and oxygen), or both oxygen and steam.
The present invention further relates to an inclusion-free silica substrate formed by the above process. The silica substrate is formed with a thickness that is about twice the thickness of the monocrystalline silicon substrate so that the silica substrate can be, for example, about 20-100 xcexcm thick.
The present invention also relates to a method for forming a silica substrate that comprises steps for providing a monocrystalline silicon substrate, heating the silicon substrate to an elevated temperature in the range of 700-1200xc2x0 C., and contacting two major surfaces of the heated silicon substrate with an oxidizing ambient at an elevated pressure in the range of 5-50 atmospheres for sufficient time to convert the entirety of the silicon substrate into silica. The monocrystalline silicon substrate used as a starting material for forming the silica substrate can have a thickness in the range of 10-50 microns, with the resulting silica substrate being about twice as thick. When using a conventional high-pressure oxidation system, the elevated temperature can be in the range of 900-1000xc2x0 C., and the elevated pressure in the range of 10-25 atmospheres. The oxidizing ambient can comprise oxygen, steam, or a combination thereof.
Finally, the present invention relates to a method for converting at least a portion of a monocrystalline silicon substrate into silica, comprising steps for heating the silicon substrate to an elevated temperature in the range of 700-1200xc2x0 C., and contacting a pair of opposing surfaces of the monocrystalline silicon portion with an oxidizing ambient at an elevated pressure in the range of 5-50 atmospheres for sufficient time to convert the entire thickness of the monocrystalline silicon portion into silica. The monocrystalline silicon portion can be defined by a step for forming a mask (e.g. comprising silicon nitride) over each major surface of the silicon substrate.